Positional effects of second-sphere amide pendants on electrochemical CO reduction catalyzed by iron porphyrins.

The development of catalysts for electrochemical reduction of carbon dioxide offers an attractive approach to transforming this greenhouse gas into value-added carbon products with sustainable energy input. Inspired by natural bioinorganic systems that feature precisely positioned hydrogen-bond donors in the secondary coordination sphere to direct chemical transformations occurring at redox-active metal centers, we now report the design, synthesis, and characterization of a series of iron tetraphenylporphyrin () derivatives bearing amide pendants at various positions at the periphery of the metal core. Proper positioning of the amide pendants greatly affects the electrocatalytic activity for carbon dioxide reduction to carbon monoxide. In particular, derivatives bearing proximal and distal amide pendants on the position of the phenyl ring exhibit significantly larger turnover frequencies (TOF) compared to the analogous -functionalized amide isomers or unfunctionalized . Analysis of TOF as a function of catalyst standard reduction potential enables first-sphere electronic effects to be disentangled from second-sphere through-space interactions, suggesting that the -functionalized porphyrins can utilize the latter second-sphere property to promote CO reduction. Indeed, the distally-functionalized -amide isomer shows a significantly larger through-space interaction than its proximal -amide analogue. These data establish that proper positioning of secondary coordination sphere groups is an effective design element for breaking electronic scaling relationships that are often observed in electrochemical CO reduction.